Method for dynamic and selective FD-DSDF transmission of a digital signal for a MARC system with a full-duplex relay, and corresponding program product and relay device

ABSTRACT

Some embodiments relate to a full-duplex relay for a telecommunications system comprising a plurality of sources, the relay, and a destination. The relay comprises a decoder which estimates for each source a code word xS,t encoding a K-bit message uS,t, from received blocks corresponding to successive code words xS,t transmitted simultaneously by the sources during T time slots, such that at each time slot t, t=1, . . . , T, a code word xS,t comprises B blocks xS,t(1), xS,t(2), . . . , xS,t(B) of which the first can be decoded independently of the other blocks. It also comprises a decision component which enables the relay to detect messages that have been decoded without error and to take a decision and an encoder and transmitter which encode a signal and transmit it to the destination, which signal is representative only of messages that have been decoded without error. After each received block, the encoding and transmission unit is under the control of the decision component.

RELATED APPLICATIONS

This application is the U.S. National Phase of Application No.PCT/FR2015/051728 entitled “METHOD FOR DYNAMIC AND SELECTIVE FD-DSDFTRANSMISSION OF A DIGITAL SIGNAL FOR A MARC SYSTEM WITH A FULL-DUPLEXRELAY, A CORRESPONDING PROGRAM PRODUCT AND RELAY DEVICE” filed Jun. 25,2015, which designated the United States, and which claims the benefitof French Application No. 1456104 filed Jun. 27, 2014.

FIELD OF THE INVENTION

In general manner, the present invention relates to the field of digitalcommunications. More precisely, the field of the invention is that oftransmitting coded data in a multiple-access relay channel (MARC)network. A MARC network is a telecommunications system that, for a givendestination, comprises at least three nodes: at least two transmittersand a relay. More precisely, the invention relates to relaying, and itseeks to improve the quality of data transmission, and in particular toimprove the performance of error-correcting decoding in a (destination)receiver. The relaying is performed by a relay that co-operates with thesources in order to obtain communication that is more reliable betweenthe sources and the destination.

The invention relates particularly, but not exclusively, to transmittingdata via mobile networks, e.g. for real-time applications. It appliesmore particularly to so-called “full-duplex” (FD) relays with linksbetween the nodes that may equally well be non-orthogonal (withinterference), semi-orthogonal (with some interference), or orthogonal(with no interference). In this application, a link is a communicationchannel between two or more nodes, and it may be physical or logical.When the link is physical, it is generally referred to as a channel.

PRIOR ART

Networks, and in particular mobile networks, are looking for significantimprovements in terms of capacity, reliability, consumption, etc. Thetransmission channel of a mobile network has the reputation of beingdifficult, and leads to transmission reliability that is relativelypoor. Over the last few years, considerable progress has been achievedin terms of encoding and modulation, in particular concerningconsumption and capacity. Specifically, in a mobile network where aplurality of transmitters/receivers share the same resources (time,frequency, and space) it is necessary to reduce transmission power asmuch as possible.

Such reduction goes against the coverage, and thus against the capacity,of the system, and more generally against its performance.

In order to increase coverage, to make communication more reliable, andmore generally to improve performance, one approach consists in relyingon relays for increasing spectrum efficiency (coding gain) and thusimproving the transmission efficiency and the reliability of systems(diversity gain). The basic topology of MARC systems as shown in FIG. 1is such that the resources, nodes S₁ and S₂, broadcast their codedinformation sequences for the attention of the relay R and of thedestination D. The relay decodes the signals received from the sourcesS₁ and S₂ and it re-encodes these jointly while adding its ownredundancy so as to create a spatially-distributed network code as seenfrom the receiver D. At the destination D, the decoding of the threespatially-distributed coded sequences, comprising the two codedsequences received directly from the sources S₁ and S₂ together with thecoded sequence coming from the relay, relies on joint channel andnetwork decoding algorithms.

Network coding is a form of cooperation in which the nodes of thenetwork share not only their own resources (power, bandwidth, etc.) butalso their computation capacity, in order to create a distributed codethat becomes more and more powerful as the information propagatesthrough the nodes. It gives rise to substantial improvement in terms ofdiversity and of encoding, and thus in terms of transmissionreliability.

A distinction is made between two types of operation for the relay:half-duplex mode and full-duplex mode. The invention applies tofull-duplex relays.

In full-duplex mode, the relay receives new information blocks from bothsources and simultaneously transmits to the destination informationbased on the previously received blocks; the relay can thus receive andtransmit simultaneously on the same frequency band or on differentbands. In comparison with a half-duplex relay, a full-duplex relay makesit possible to achieve greater capacity.

Patent application WO 2011/033237 relates to a MARC system with afull-duplex relay implementing a transmission protocol of the Markovchain type. In that protocol, the destination observes block-codedinformation coming from the two sources S₁, S₂ superposed on thepreceding block-coded information coming from the relay. Thereafter, theprocessing on reception needs to make joint use of a plurality ofsuccessive blocks. In the protocol used, as shown in FIG. 2, each sourcehas T K-bit messages for transmitting during a time slot t, t ϵ{1, . . ., T}. At the end of each time slot t, where t ϵ{1, . . . , T} the relayR₁ attempts to decode the messages from the sources and to transmit theresult of a deterministic function of the decoded messages during thetime slot t+1. The protocol thus requires T+1 time slots in order totransmit the T messages. The destination attempts to decode the messagesfrom the sources at the end of time slot T+1 by using the signalstransmitted throughout the period of the T+1 time slots. Such a protocolis complex to implement when the relays perform decoding of flexibletype with log likelihood ratios (LLRs) being transmitted to thedestination.

Patent application WO 2012/022905 relates to a MARC system implementinga relaying protocol that does not transmit words that have been decodederroneously by using a “selective decode and forward” (SDF) technique.Error detection in the relay is based on verifying a cyclic redundancycheck (CRC) included in each source message. That protocol serves toreduce the complexity of decoding at the destination and at the relay,while conserving high capacity when the protocol is implemented with afull-duplex (FD) relay. With reference to FIG. 2, if the relay hascorrectly decoded the sources at the end of the time slot t, then duringthe time slot t+1 it transmits a payload signal to help both sources. Ifonly one source is correctly decoded, then the signal is representativeof that source only. If neither source is correctly decoded, then therelay remains silent.

MAIN CHARACTERISTICS OF THE INVENTION

The invention relates to transmitting a digital signal in a networkhaving at least four nodes comprising two transmitters, a relay, and adestination, the method enabling transmission capacity to be optimizedwhen the relay is a full-duplex relay, which is thus capable ofreceiving and transmitting simultaneously, while limiting the complexityof decoding at the relay and at the destination.

The invention provides a relaying method implemented by the full-duplexrelay for a telecommunications system comprising a plurality of sources,the relay, and a destination. The method comprises:

-   -   a decoding step for estimating for each source a code word        x_(S,t) from received blocks corresponding to successive code        words x_(S,t) transmitted simultaneously by the sources during T        time slots, such that, at each time slot t, t=1, . . . , T, a        code word x_(S,t) comprises B blocks x_(S,t) ⁽¹⁾, x_(S,t) ⁽²⁾, .        . . , x_(S,t) ^((B)) of which the first can be decoded        independently of the other blocks, a code word x_(S,t) encoding        a K-bit message u_(S,t);    -   a step of the relay detecting messages that have been decoded        without error and taking a decision; and    -   a step of encoding a signal and of transmitting it to the        destination, which signal is representative only of messages        that have been decoded without error.

The method is such that, after each received block, the encoding andtransmission are under the control of the step of detecting messagesthat have been decoded without error and of taking a decision incompliance with a selection scheme.

The invention also provides a full-duplex relay for a telecommunicationssystem comprising a plurality of sources, the relay, and a destination.The relay comprises:

-   -   a decoding unit for estimating for each source a code word        x_(S,t) from received blocks corresponding to successive code        words transmitted simultaneously by the sources during T time        slots, such that, at each time slot t, t=1, . . . , T, a code        word x_(S,t) comprises B blocks x_(S,t) ⁽¹⁾, x_(S,t) ⁽²⁾, . . .        , x_(S,t) ^((B)) of which the first can be decoded independently        of the other blocks, a code word x_(S,t) encoding a K-bit        message u_(S,t);    -   a decision unit enabling the relay to detect messages that have        been decoded without error and to take a decision; and    -   an encoding and transmission unit for encoding a signal and for        transmitting it to the destination, which signal is        representative only of messages that have been decoded without        error.

After each received block, the encoding and transmission unit is underthe control of the unit for detecting errors and for taking a decisionin compliance with a selection scheme.

Thus, the relay forms part of a MARC system that has at least twosources (transmitters) and the destination. The MARC system may havemore than two transmitters (sources). The transmitters transmitsimultaneously to the same receiver (destination), thus enabling maximumuse to be made of the common spectrum resource.

The transmission by the sources may take place simultaneously over thesame radio resource (time and frequency), which makes it possible tomaximize use of the common spectrum resource; the source-relay links arethen not orthogonal. There is thus interference between the signalsreceived by the relay and by the destination as a result of the sourcesignals being superposed during transmission, firstly between thesources and the relay and secondly between the sources and thedestination (the receiver).

In an embodiment, the sources transmit simultaneously over the sameradio resource, and the decoding step at the relay is iterative andcomprises joint detection and joint decoding. The joint detection anddecoding in the relay make it possible to separate the streamstransmitted simultaneously by the transmitters.

When the sources transmit simultaneously but over different spectrumresources, the relay does not need the iterative joint detection anddecoding step. Under such circumstances, the relay can decode themessages from the sources on the basis of sequences received withoutinterference between the sources. Under such circumstances, thesource-relay links are orthogonal.

The relay may optionally receive and transmit on different frequencybands. Such band separation is more particularly advantageous for a MARCsystem having only one relay. Such a configuration makes it possible touse a receive antenna that is dedicated and it serves above all tosimplify processing on reception by limiting disturbances due to therelay transmitting simultaneously in the same band.

The use of encoding at the source (transmitters) whereby the first blockcan be decoded independently of the other blocks avoids the need for anadditional time slot; T messages are transmitted for each transmitter inT time slots. The encoding at the source may be encoding of the finiteincremental redundancy type.

The encoding at the relay, which includes network coding and channelcoding, enables all of the dispersed transmitters to benefit from thecoding gain of a spatially distributed network code without decreasingspectrum efficiency. This makes it possible in the receiver to implementiterative decoding that relies on redundancy generated in the relaywithout requiring power to be increased in the transmitters in order toachieve an increase in the coverage of the system and in spectrumefficiency.

By not transmitting the messages that are detected with error, thetransmission protocol avoids any propagation of transmission errors asintroduced more particularly when the links from the transmitters to therelay are not very reliable. Interleaving messages that have beendetected without error is a known technique that is necessary forachieving joint channel decoding at the destination of the signals fromthe transmitters and of the signal from the relay.

The protocol contributes to reducing energy consumption by combatingerror propagation and by effectively combating interference (when it iscooperating, the relay always transmits payload information).

The protocol makes it possible to reach a maximum data rate bycontrolling the encoding and the transmission without waiting for themessages from all of the transmitters to be decoded without error andwithout waiting for the end of the time slot or without waiting for thelast time slot. Thus, during a time slot, the relay transmits redundancyrelating to a message that has been decoded without error during thesame time slot. As a result, additional redundancy is transmitted almostimmediately to the destination, which redundancy is determined by therelay, even while the relay continues to decode messages coming from theother transmitters, which is not possible with a relay of thehalf-duplex type. The protocol gives particularly good performance sinceby making full use of the ability of a full-duplex relay to receive andtransmit simultaneously, it does not require a threshold concerning anumber of time slots in order to decide whether to switch betweendetecting messages and encoding messages that have been detected withouterror.

When the various messages transmitted by the transmitters are mutuallyindependent, the protocol avoids introducing pointless latency in theprocessing on reception of these messages, as can happen in the priorart by waiting for the end of the time slot T+1 before attempting todecode all of the messages, or by the relay waiting for a certain numberof time slots to elapse before switching to encoding. Instead of beingsilent during a time slot t ϵ{2, . . . , T} in the absence of anymessage that has been decoded without error at the end of timeslot t−1,the relay can provide help for any message that has been decoded withouterror previously, i.e. during the time slots 1, . . . , t−1. Thetransmission protocol of the invention thus ensures full use is made ofthe full-duplex capabilities of a relay, and leads to optimizedtransmission capacity.

The relays may equally well be stationary relays or mobile relays. Giventhe density of communications to be transmitted in densely populatedzones, the number of relays may be large, and much greater than two.Specifically, in order to cover such zones, stationary relays may beused in preference to base stations, which are of a cost that can beconsiderably greater. Alternatively, it is possible to use mobilerelays. Such mobile relays are typically mobile terminals.

In an embodiment, messages that have been decoded without error aredetected by means of a CRC type code included in each K-bit messageu_(S,t).

In an embodiment, the messages that have been decoded without error arestored.

By storing the messages that have been decoded without error, a relaycan make use several times over of the same message that has beendecoded without error, i.e. it can use it during a plurality of timeslots, in order to generate the signal going to the destination.

In an embodiment, in the absence of a message that has been decodedwithout error during a current time slot t, the error detection anddecision taking step allows a message that has been decoded withouterror during a preceding time slot to be encoded and transmitted.

Thus, instead of being silent during the time slots following thecurrent time slot t while waiting to decode correctly the message fromanother source, the relay can continue to transmit the redundancydetermined on the message from the source that has been decoded withouterror during an earlier time slot 1, . . . , t−1.

In an embodiment, the decision taken allows a message to be encoded andtransmitted as soon as it has been decoded without error.

Thus, the relay of the invention transmits redundancy to the destinationwithout delay, i.e. without waiting for the end of the time slot, whichredundancy relates to the source that has been decoded without error,thus providing help to the destination in decoding the same source. Thisgain provided for one source can be beneficial to the other sources.Specifically, by releasing decoding means both in the relay and in thedestination in order to concentrate on the sources that have not yetbeen decoded correctly, the protocol increases the probability ofdecoding all of the sources without error.

In an implementation, after each received block, detection and decodingare under the control of the step of detecting messages that have beendecoded without error and of taking a decision in compliance with aselection scheme.

In this implementation, detection and decoding may be adapted as afunction of error detection.

In an implementation, if all the messages are decoded without errorduring a current time slot t, then the step of detecting messages thathave been decoded without error and of taking decisions stops detectionand decoding until the end of the current time slot and allows encodingand transmission.

More particularly, this implementation makes it possible to emphasisethe reduction in energy consumption by avoiding any pointlessexpenditure of the detection and decoding means. It also makes itpossible to concentrate the energy of the relay on other means.

In an implementation, the selection scheme is such that, at each currenttime slot t, the transmitted signal is representative of the messagesdecoded without error up to the current time slot t.

Thus, the destination can benefit from a plurality of differentredundancies for a single message correctly decoded by the relay. Thedestination making use of these various occurrences contributes toincreasing the probability of correctly decoding the received messages.

In an implementation, the selection scheme is such that, during eachcurrent time slot t, the transmitted signal is representative of themessages that have been decoded without error solely during the currenttime slot t.

This implementation has the advantage of being simple since it requireslittle signalling.

In an implementation, the encoding and transmission step includesinterleaving for each source for which a message has been decodedwithout error prior to network coding.

Interleaving at the input to network coding makes it possible for thedestination to have a structure similar to parallel concatenation(similar to a distributed turbo-code). As a function of the way thenetwork coding is decoded at the destination, the interleaving mayoptionally be different among the sources.

In an implementation, the encoding and transmission step comprisesnetwork coding followed by first interleaving, by channel coding, and bysecond interleaving distinct from the first.

The second interleaving makes it possible to give a certain signature tothe signal transmitted by the relay, thus making it possible todistinguish it from the signals transmitted simultaneously by thetransmitters. After separating the signal transmitted by the relay, thefirst interleaving, which may optionally be variable as a function ofthe block, makes it possible to distinguish (in the statisticalindependence meaning) between network coding and channel coding.

In an embodiment, the full-duplex relay further comprises a unit forstoring the messages that have been decoded without error.

In an embodiment, the full-duplex relay is such that, after eachreceived block, the decoding unit is under the control of the unit fordetecting errors and for taking a decision in compliance with aselection scheme.

In an embodiment, the full-duplex relay is such that the encoding andtransmission unit includes one interleaver per source for which amessage has been decoded without error at the input of a network code.

In an embodiment, the full-duplex relay is such that the encoding andtransmission unit includes a network code followed by a firstinterleaver, by a channel encoder, and by a second interleaver distinctfrom the first.

The advantages of the full-duplex relay are the same as the advantagesof the relaying method. Consequently, they are not described in detail.

The invention also provides a MARC system in which the relay is afull-duplex relay of the invention.

The invention also provides a reception method for a receiver of atleast one MARC system for performing a relaying method of the invention.The reception method comprises:

-   -   joint detection and decoding of blocks coming from the sources        and of messages coming from the relay, the detection and        decoding being performed iteratively at the end of each time        slot t such that t ϵ{Q, . . . , T} over the duration of a        sliding window of length Q, Q ϵ{1, . . . , T}, with propagation        of probabilities between the iterations in order to estimate Q        messages for each source.

The reception method is such that the decoding of messages from therelay is configured at each sub slot in compliance with signallinginformation coming from the relay indicating whether the relay iscooperating on this block b, b=1, . . . B, and such that probabilitypropagation is configured at each block b, b=1, . . . B, in compliancewith signalling information coming from the relay indicating theselection scheme of the relay.

The invention also provides a receiver for at least one MARC system forperforming a relaying method of the invention. The receiver comprises:

-   -   a detection and decoding unit for joint detection and decoding        of blocks coming from the sources and of messages coming from        the relay, the detection and decoding being performed        iteratively at the end of each time slot t such that t ϵ{Q, . .        . , T} over the duration of a sliding window of length Q, Q ϵ{1,        . . . , T}, with propagation of probabilities between the        iterations in order to estimate Q messages for each source.

The receiver is such that the decoding of messages from the relay isconfigured at each sub slot in compliance with signalling informationcoming from the relay indicating whether the relay is cooperating onthis block b, b=1, . . . B, and such that probability propagation isconfigured at each block b, b=1, . . . B, in compliance with signallinginformation coming from the relay indicating the selection scheme of therelay.

The invention also provides a method of transmitting a digital signalfor a telecommunications system comprising a plurality of sources ({S₁,S₂, . . . , S_(M)}), the relay, and a destination implementing aspatially distributed network code, the method comprising for eachsource:

-   -   a step of encoding messages u_(S,t) including respective CRCs        into code words c_(s,t); and    -   a step of transmitting the code words c_(s,t) during T time        slots to the relay and to the destination.

The transmission method is such that the encoding is of finiteincremental redundancy type and, at each sub slot b=1, 2, . . . , B of atime slot, it delivers a block c_(S,t) ^((b)) such that the B successiveblocks {c_(S,t) ^((b)): 1≤b≤B} form the code word c_(s,t), such that thefirst block can be decoded independently of the other blocks, and suchthat the following blocks are parity bits that add redundancy to thefirst block.

The invention also provides a transmitter for transmitting a digitalsignal for a telecommunications system comprising a plurality ofsources, the relay, and a destination implementing a spatiallydistributed network code, comprising for each transmitter:

-   -   an encoder for encoding messages u_(S,t) having respective CRCs        into code words c_(s,t); and    -   a transmitter for transmitting the code words c_(s,t) during T        time slots to the relay and to the destination.

The transmitter is such that the encoder is of finite incrementalredundancy type and, at each sub slot b=1, 2, . . . , B of a time slot,it delivers a block c_(S,t) ^((b)) such that the B successive blocks{c_(S,t) ^((b)): 1≤b≤B} form the code word c_(s,t), such that the firstblock can be decoded independently of the other blocks, and such thatthe following blocks are parity bits that add redundancy to the firstblock.

In a preferred implementation, the steps of the relaying method aredetermined by instructions of a relaying program incorporated in one ormore electronic circuits such as chips, which themselves may be arrangedin electronic devices of the MARC system. The relaying method of theinvention may equally well be performed when the program is loaded intoa calculation member such as a processor or the equivalent with itsoperation then being controlled by executing the program.

Consequently, the invention also applies to a computer program, inparticular a computer program on or in a data medium, and suitable forperforming the invention. The program may use any programming language,and it may be in the form of source code, object code, or codeintermediate between source code and object code, such as in a partiallycompiled form, or in any other desirable form for implementing a methodof the invention.

The data medium may be any entity or device capable of storing theprogram. For example, the medium may comprise storage means such as aread only memory (ROM), for example a compact disk (CD) ROM or amicroelectronic circuit ROM, or indeed magnetic recording means, e.g. ahard disk, or universal serial bus (USB) stick.

Alternatively, the data medium may be an integrated circuit in which theprogram is incorporated, the circuit being adapted to execute or to beused in the execution of the method in question.

Furthermore, the program may be converted into a transmissible form suchas an electrical or optical signal that can be conveyed via anelectrical or optical cable, by radio, or by other means. The program ofthe invention may in particular be downloaded from a network of theInternet type.

The invention thus also provides a computer program on a data medium,said program including program instructions adapted to performing amethod of relaying a digital signal of the invention, when said programis loaded and executed in a relay for a MARC system for performing therelaying method.

The invention also provides a data medium including program instructionsadapted to performing a method of relaying a digital signal of theinvention, when said program is loaded and executed in a relay for aMARC system for performing the relaying method.

LIST OF FIGURES

Other characteristics and advantages of the invention appear moreclearly on reading the following description of implementations givenmerely as illustrative and nonlimiting examples, and from theaccompanying drawings, in which:

FIG. 1 is a diagram showing the basic topology of MARC systems;

FIG. 2 is a diagram showing a prior art cooperation protocol in whichthe sources transmit during T time slots, and the relay co-operatesafter the end of each time slot, thereby leading to cooperation thatrequires T+1 time slots;

FIG. 3 is a diagram showing the topology of a MARC system having Ntransmitters, one relay, and one destination;

FIG. 4 is a diagram of an implementation of the processing implementedby a source of the invention;

FIG. 5 is a diagram of an embodiment of a relay of the invention;

FIG. 6 is a flowchart of the method of the invention implemented by therelay R;

FIG. 7 is a diagram of an embodiment of the detection and decoding unit(DDU) of a relay of the invention;

FIG. 8 is in the form of a factor graph showing the iterative structureof the DDU used for decoding the source S during the time slots t ϵ{1, .. . , T};

FIG. 9 is a diagram of an embodiment of the encoding and transmissionunit (ETU) of a relay of the invention;

FIG. 10 shows an example of cooperation by the relay R₁ of a MARC systemhaving two sources S₁ and S₂ when there is only one time slot subdividedinto B sub slots;

FIGS. 11 and 12 show examples of cooperation by the relay R₁ of a MARCsystem having two sources S₁ and S₂ during the time slots t ϵ{1, . . . ,T}, each being subdivided into B sub slots;

FIG. 13 is in the form of a factor graph showing the iterative structureof the destination decoder used for decoding the sources S₁ to S_(M)during the time slots from t−Q+1 to t.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The context of the invention is that where a plurality of sources(transmitters) S₁, . . . , S_(M) seek to send their respective messagesto a common destination D using a full-duplex relay R, as shown in FIG.3.

There is no constraint on the transmission channel; it may be subject tofast or slow fading, it may be frequency selective, and it may be amultiple-input and multiple-output (MIMO) channel. In the descriptionbelow, the nodes (sources, relays, and destination) are assumed to beaccurately synchronised, and the sources are independent (there is nocorrelation between them).

A transmission cycle is made up of T time slots. The duration of a cycledepends on the settings of the MARC system and in particular on the MAClayer of the seven-layer OSI model. At each time slot, each source has aK-bit message for transmitting. CRC type information forms part of theK-bit message transmitted by a source, and it is used to determinewhether a received message is decoded correctly. The T messages from asource may be independent of one another or they may be correlated andconstitute a frame.

The sources transmit simultaneously. The relay attempts to transmit apayload signal to the destination, which signal is representative ofestimated messages in order to provide help in communication between thesources and the destination.

F₂ is the two element Galois field, R is the real number field and C isthe complex number field.

The transmission scheme at the sources is shown in FIG. 4.

At each time slot t, t=1, . . . , T, each source S from among the Msources {S₁, S₂, . . . , S_(M)} has a message u_(S,t) made up of K bitsof information for transmission, u_(S,t)ϵF₂ ^(K). The message u_(S,t)includes a CRC type code that makes it possible to verify the integrityof the message u_(S,t).

The statistically independent sources {S₁, S₂, . . . , S_(M)} performencoding of the message u_(S,t) by means of an incremental redundancycode and they transform the message u_(S,t) into n_(S) bits writtenc_(s,t)ϵF₂ ^(n) ^(S) . The resulting code word c_(s,t) is segmented intoB redundancy blocks, each of which is transmitted during a sub slot,written c_(S,t) ^((b))ϵF₂ ^(n) ^(S,b) with b=1, 2, . . . , B. Each blockcomprises n_(S,b) bits, with

$n_{S} = {\sum\limits_{b = 1}^{B}\;{n_{S,b}.}}$The incremental redundancy code may be of the systematic type, in whichcase the information bits are included in the first block: u_(S,t)ϵc_(S,t) ⁽¹⁾. Regardless of whether or not the incremental redundancycode is of the systematic type, it is such that the first block c_(S,t)⁽¹⁾ from among of the B blocks can be decoded independently of the otherblocks. The highest coding rate at the source S is K/n_(S,1) and itcannot be greater than one, n_(S,1)>K. The minimum coding rate for asource S is K/n_(S). Each block {c_(S,t) ^((b)): 1<b≤B} after the firstblock includes parity bits that add redundancy to the first block, eachof the blocks {c_(S,t) ^((b)): 1<b≤B} being capable of being decodedjointly with the first block.

By way of example, the incremental redundancy code can be provided inpractice by means of a finite family of punctured linear codes withcompatible coding rates or of codes without coding rates that have beenmodified to operate with finite lengths: raptor code (RC), ratecompatible punctured turbo code (RCPTC), rate compatible puncturedconvolutional code (RCPCC), rate compatible low density check code(LDPC).

Each block c_(S,t) ^((b)) is interleaved by a distinct interleaverwritten π_(S) ^((b)), the code word after interleaving being writtenb_(S,t)=[b_(S,t) ⁽¹⁾, b_(S,t) ⁽²⁾, . . . b_(S,t) ^((B))]. Theinterleavers serve to combat fading, which may occur during transmissionvia the channel, and to give each source a fingerprint that makes thesources easier to separate by the relay and by the destination. Eachinterleaved portion b_(S,t) ^((b)) of the code word is modulated inorder to obtain a complex code word x_(S,t)=[x_(S,t) ⁽¹⁾, x_(S,t) ⁽²⁾, .. . , x_(S,t) ^((B))] with x_(S,t) ^((b))ϵX^(N) ^(b) , b=1, 2, . . . ,B, where X⊂C designates a complex signal of cardinal number |X|=2^(q)^(s) and where N_(b)=n_(S,b)/q_(S).

Each source S from among the M sources {S₁, S₂, . . . , S_(M)} may use aminimum coding rate K/n_(S) and a modulation order q_(s) that aredifferent from those of the other sources, in so far as the sub slots ofthe transmitted complex code words are identical between the sources:n_(S,b)/q_(S)=N_(b), b=1, 2, . . . , B.

Each source S transmits the code word x_(S,t)=[x_(S,t) ⁽¹⁾, x_(S,t) ⁽²⁾,. . . , x_(S,t) ^((B))] made up of B blocks during a time slot that issubdivided into B sub slots. Whatever the value of b, 1≤b≤B, theconcatenation (or accumulation) of the blocks 1 to b is itself a codeword since it comes from the incremental redundancy encoder.

In order to simplify reception structure at the relay and at thedestination, the sources described have interleavers, an encoder, and amodulator that do not vary as a function of the time slot t, t=1, . . ., T, but it would be equally possible for them to be variable.

The invention proposes a novel approach for the cooperation of a relayof a MARC system in order to give help to the sources that transmit,thereby obtaining an improvement in the spectrum efficiency of thetransmission while enabling the encoding to be simple and effective inthe receiver of the destination.

A relaying method of the invention is implemented by the at least onefull-duplex relay for a MARC system.

This relay R of the invention is shown in FIG. 5. The relay comprises aniterative detection and decoding unit DDU followed by a decision unit DUand by an encoding and transmission unit ETU. The detection and decodingunit DDU periodically delivers an estimated version of the decodedmessages from the sources. The decision unit DU tests the CRCs of theestimated messages supplied by the DDU in order to determine whichmessages have been decoded without error, and it decides whether therelay is to transmit and what information it should transmit in each subslot during the T time slots. The encoding and transmission unit ETUtransmits the payload signal to the destination during the cooperationstage of the relay. The information to be included in the signal isselected by the decision unit DU.

A flowchart of the method performed by the relay R is shown in FIG. 6.The method 1 comprises a decoding step 2, an error detection anddecision step 3, and an encoding and transmission step 4 for sending asignal to the destination that is representative only of messages thathave been decoded without error. The error detection and decision stepcontrols encoding and transmission and feeds the encoding in compliancewith a selection scheme after each received block. In an implementation,the error detection and decision step also controls decoding.

The decoding step 2 is performed by the detection and decoding unit DDUof the relay. This detection and decoding step supplies an estimate ofthe messages on the basis of the received words corresponding to thewords x_(S,t)=[x_(S,t) ⁽¹⁾, x_(S,t) ⁽²⁾, . . . , x_(S,t) ^((B))]transmitted by each source S from among the M sources {S₁, S₂, . . . ,S_(M)}.

In the event of simultaneous transmission over the same radio resource,the relay performs detection and decoding in joint and iterative manner.The structure of the detection and decoding unit DDU is shown in FIG. 7and in simplified form for the source in FIG. 8. The unit DDU is ofiterative structure constituted by a soft-in soft-out multiuser detector(SISO MUD) followed by as many soft channel decoders in parallel asthere are sources {S₁, S₂, . . . , S_(M)}. For a block b, b ϵ{1, 2, . .. , B}, received during the time slot t, t=1, . . . , T, the receivedsequence is expressed in the following form:

$y_{R,t}^{(b)} = {{\sum\limits_{i = 1}^{M}\;{h_{S_{i},R,t}x_{S_{i},t}^{(b)}}} + n_{R,t}^{(b)}}$where h_(S) _(i) _(,R,t)ϵC represents the channel gain between thesource S_(i) and the relay R and where n_(R,t) ^((b))ϵC^(N) ^(b)represents additive Gaussian noise. The DDU uses the current block by_(R,t) ^((b)) and all of the previously received blocks y_(R,t) ⁽¹⁾,y_(R,t) ⁽²⁾, . . . , y_(R,t) ^((b−1)) in order to obtain an estimate ofthe messages û_(S) ₁ _(,t), û_(S) ₂ _(,t), . . . , û_(S) _(M) _(,t) fromthe sources.

At the end of receiving the first block, at each iteration, themultiuser detector MUD calculates in the form of a log likelihood ratio(LLR) the a posteriori probability (APP) of each interleaved and codedbit from each source S:

${{{{LAPPR}\text{:}\mspace{14mu}{\Lambda\left( \left\lbrack b_{S,t}^{(1)} \right\rbrack_{k} \right)}} \cong {{\log\left( \frac{\Pr\left( {\left\lbrack b_{S,t}^{(1)} \right\rbrack_{k} = {1❘y_{R,t}^{(1)}}} \right)}{\Pr\left( {\left\lbrack b_{S,t}^{(1)} \right\rbrack_{k} = {0❘y_{R,t}^{(1)}}} \right)} \right)}\mspace{14mu} k}} = 1},2,\ldots\mspace{14mu},n_{S,1}$in which expression n_(S,1) is the number of bits transmitted by eachsource S in the first block. This LAPPR may be written in the followingform:Λ([b _(S,t) ⁽¹⁾]_(k))=Λ_(a)([b _(S,t) ⁽¹⁾]_(k))+Λ_(e)([b _(S,t)⁽¹⁾]_(k))in which expression Λ_(a)([b_(S,t) ⁽¹⁾]_(k)) is the logarithm of the apriori probability ratio (LAPR) supplied by the SISO channel decoder ofeach source S at the preceding iteration, and Λ_(e)([b_(S,t) ⁽¹⁾]_(k))is the extrinsic information associated with the bit [b_(S,t) ⁽¹⁾]_(k).

The vector Λ_(e)([b_(S,t) ⁽¹⁾]_(k)) is deinterleaved by a deinterleaver[π_(S) ⁽¹⁾]⁻¹, and the output Λ_(a)([c_(S,t) ⁽¹⁾]_(k)) is the logarithmof the a priori probability ratio of the first block. The SISO channeldecoder of the source S uses Λ_(a)([c_(S,t) ⁽¹⁾]_(k)) to provide adecoded version û_(S,t) of the message u_(S,t) to the DU and to provideextrinsic information Λ_(e)[c_(S,t) ⁽¹⁾] of the bits. The vector of thisextrinsic information is interleaved by the interleaver π_(S) ⁽¹⁾. Theoutput Λ_(a)[b_(S,t) ⁽¹⁾] from the interleaver (after SISO decoding thesource 1) is looped back to the input of the SISO multiuser detectorMUD_(t) ^((b)) and marks the end of an iteration.

The iterations of the DDU come to an end, either because they havereached a maximum number of iterations, or else under the control of theerror detection and decision step. For example, the error detection anddecision step does not allow any further detection and decoding if allof the sources have been detected without error before the end of thetime slot.

At the end of receiving a new block b, the switches corresponding tothis block b and to all of the preceding blocks are switched. Theprocedure described for the first block is repeated, but taking intoconsideration b _(S,t) ^((b))=[(b_(S,t) ⁽¹⁾, b_(S,t) ⁽²⁾, . . . ,b_(S,t) ^((b))] instead of [b_(S,t) ⁽¹⁾].

At the end of receiving the block B^(e), all of the switches of the DDUare switched and the procedure described for the first block isrepeated, but taking into consideration b _(S,t) ^((B))=[b_(S,t) ⁽¹⁾,b_(S,t) ⁽²⁾, . . . , b_(S,t) ^((B))] instead of [b_(S,t) ⁽¹⁾].

FIG. 8 shows the principle of multiuser detection and of joint decoding,in the form of a factor graph. The variable nodes are represented bycircles and the constraint nodes by squares, with a constraint nodebeing a function of the variable nodes that are attached thereto. Thus,the constraint node π_(S) ^((b)) represents the constraint functionƒ(b_(S,t) ^((b)), c_(S,t) ^((b))) that requires b_(S,t) ^((b)) andc_(S,t) ^((b)) to be linked by the interleaver π_(S) ^((b)), i.e.b_(S,t) ^((b))=π_(S) ^((B))c_(S,t) ^((b)) or c_(S,t) ^((b))=[π_(S)^((b))]⁻¹ b_(S,t) ^((b)). The constraint node MUD_(t) ^((b)) is linkedto the variable nodes b_(S) ₁ _(,t) ^((b)), . . . , b_(S) _(M) _(,t)^((b)) and to the observation t_(R,t) ^((b)). Since we are payingattention to the variable node u_(S,t), only the variable node b_(S,t)^((b)) is shown. The rules of the “sum-product algorithm” [1] enablebeliefs to be calculated from a constraint node to a variable node bytaking account of the incident beliefs of the other variable nodes; thiscorresponds to activating multiuser detection for the constraint nodeMUD_(t) ^((b)) or to activating decoding for the “SISO source S decoder”constraint node as shown in FIG. 7. Thus, in this example, the beliefsor probability messages (for each bit) are the extrinsic information inthe form of extrinsic log likelihood ratios (LLRs).

Thus, during time slot t and block b, processing runs as follows. Foreach source S ϵ{S₁, . . . , S_(M)}, the DDU runs the following steps:

1. If the MUD_(t) ^((l)) l=1, . . . ,b are activated, they generatetheir beliefs concerning the values of the variable nodes b _(S,t)^((b)) (these beliefs are generally in the form of vectors of LLRvalues).

2. The beliefs concerning b _(S,t) ^((b)) are converted into beliefsconcerning c _(S,t) ^((b))=[c_(S,t) ⁽¹⁾, c_(S,t) ⁽²⁾, . . . , c_(S,t)^((b))] by the deinterleavers [π_(S) ^((l))]⁻¹ l=1, . . . , b.

3. The beliefs concerning c _(S,t) ^((b)) are passed to the SISO channeldecoder of the source S. In return, it generates its belief concerningthe variable nodes u_(S,t) and c _(S,t) ^((b)).

4. The beliefs concerning u_(S,t) are passed to the DU.

5. If the DU allows the DDU to continue, then the belief concerning c_(S,t) ^((b)) is converted into belief concerning b _(S,t) ^((b)) by theinterleaver π_(S) ^((l)) l=1, . . . , b.

6. The beliefs concerning b _(S,t) ^((b)) are passed to the MUD_(t)^((l)) l=1, . . . , b.

7. Repeat steps 1 to 6.

Step 3 of detecting messages that have been decoded without error and ofthe relay taking a decision is performed by the DU. During each timeslot t=1, . . . , T, step 3 detects errors in the estimated messagesû_(S) _(1,t) , û_(S) _(2,t) , . . . , û_(S) _(M) _(,t) at the end ofeach sub slot b ϵ{1, 2, . . . , B}. In an implementation, errordetection is performed by making use of CRC type information included inthe first of the B blocks coming from the sources.

After each received block, encoding and transmission are controlled asfollows:

1. In the absence of any message that has been decoded without error,the DU uses a selection scheme to decide which messages decoded duringthe preceding time slots need to be helped. For example, it is possibleto help only the last message that has been decoded correctly, or by wayof example it is possible to help any set of messages that have beendecoded without error.

2. If some sources have been decoded without error, the unit allows helpfor these sources, i.e. it allows the ETU to encode and transmit forthese sources.

3. If all of the sources are decoded without error, the DU causes theETU to help all of the sources to the end of the current time slot, i.e.the error detection and decision step allows the messages from all ofthe sources to be encoded and transmitted up to the end of the currenttime slot. In an implementation, the DU causes the DDU to stopprocessing the received signals, i.e. the error detection and decisionstep prevents decoding until the end of the current time slot.

The selection scheme is adapted as a function of the storage capacity ofthe relay, and a function of the size of the sliding window when asliding window is used by the destination, and as a function of thequantity of additional signalling needed for the destination. Among thevarious possible selection schemes, it is possible to distinguish thefollowing two extreme schemes.

A first scheme in which help is allowed for any set of messages from anysources that have been correctly decoded during the preceding time slots(the messages that satisfy û_(S,i)=u_(S,i) for every i=1, . . . , t−1and for every source ϵ{S₁, . . . , S_(M)}). This set is taken at theinput to the network coding (at a given time slot and a given sub slot,the relay information may be a function of any message transmitted bythe sources). This scheme is compatible with so-called “backward”decoding at the destination providing a sliding decoding window is usedthat is of size Q=T.

A second scheme in which help may be given only to messages that havebeen decoded without error previously during the same time slot.Compared with the preceding scheme, this scheme requires a minimum ofsignalling. Storage size is small: at most MK bits. This scheme iscompatible with decoding at the destination using a sliding decodingwindow that is of size Q=1.

Step 4 of encoding and transmission to the destination is performed bythe encoding and transmission unit ETU shown in FIG. 9. This encodingand transmission step encodes the messages that have been decodedwithout error in order to transmit a signal that is representative onlyof those messages that have been decoded without error. The relayencodes jointly only those messages that have been decoded without errorand it adds its own redundancy so as to create a network code.

This encoding and transmission step is under the control of the errordetection and decision taking step that optionally allows encoding andthat selects the sources to which help is to be given.

FIG. 10 shows the B sub slots of the first time slot. Each sub slot bcorresponds to a data block b. The figure shows an example of selectionwhen T=1. In this selection, the relay detects the source message S₁without error at the end of the second sub slot and it detects thesource message S₂ without error at the end of the fifth sub slot. Giventhat the relay is a full-duplex relay, it can provide help to the sourceS₁ during the sub slots b=3, 4, 5 (it thus being understood that it cantransmit a representative signal) while simultaneously continuing tolisten (i.e. to receive). At the end of the fifth sub slot, the relaystops listening since it has correctly decoded all of the messages fromthe sources and it provides help to both sources by transmitting asignal resulting from network coding of the messages from S₁ and S₂.

FIG. 11 shows an example of selection when t=1, . . . , T. During thefirst time slot t=1, the relay is capable of decoding without error S₁and then S₂ at the end of the sub slots 3 and 5 respectively. During thetime slot t=2 the relay is capable of decoding without error the messagefrom S₂ at the end of sub slot 4. Under such circumstances, during thesub slots 1, 2, 3, and 4 of the time slot t=2, the relay continues toprovide help to the messages from S₁ and S₂ of the first time slot. Asfrom sub slot 5 of time slot t=2, the relay begins to provide help forthe message from S₂ of the second time slot until a new message iscorrectly decoded, and so on. The selection scheme of FIG. 12 isidentical to the scheme of FIG. 11 except that the relay does nottransmit any more after time slot t=T. Thus, the protocol of theinvention makes it possible to save a time slot compared with the priorart by giving help immediately to the message that has been decodedwithout error, without waiting for the end of time slot t=T. Theprotocol is thus more efficient concerning channel occupancy and thustransmission capacity.

During each time slot t ϵ{1, . . . , T} and at the end of each sub slotb ϵ{1, . . . , B}, as a general rule the unit DU orders the unit ETU totransmit a new signal generated from a new set J_(R,t) ^(b) of sourcemessages that have been decoded without error during different timeslots. At the input of the network encoder, the relay interleaves eachmessage of this set J_(R,t) ^(b) by a distinct interleaver π_(R,t,|J)_(R,t) _(b) _(|) ^(b) between the blocks b and between the messages ofthe set J_(R,t) ^(b).

By way of example, the network encoder is an exclusive OR on theinterleaved messages u′_(J) _(R,t) _(b) _((|J) _(R,t) _(b) _(|)). Theoutput from the network encoder is a message having K information bits,written u_(R,t) ^(b)ϵF₂ ^(K). The message u_(R,t) ^(b) is interleaved byan interleaver written ψ_(R,t) ^(b). The interleaved message, writtenũ_(R,t) ^(b) is converted by a channel encoder followed by a functionfor selecting bits in a sequence c_(r,t) ^(b)ϵF₂ ^(n) ^(r) from n_(R)bits. The sequence c_(R,t) ^(b) is interleaved by a distinct interleaverwritten Π_(R,t) ^(b) and then modulated to obtain the complex sequencex_(R,t) ^(b)ϵX^(N) ^(b) in which X⊂C represents a complex signal ofcardinal number |X|=2^(q) ^(R) .

At the end of the time slots, the destination attempts to extract themessages from each source and from the relay. The sequence receivedduring time slot t ϵ{1, . . . , T} and during block b ϵ{1, 2, . . . , B}is as follows:

$y_{D,t}^{(b)} = {{\sum\limits_{i = 1}^{M}\;{h_{S_{i},D,t}x_{S_{i},t}^{(b)}}} + {h_{R,D,t}x_{R,t}^{(b)}1_{\{{J_{R,t}^{b} \neq \phi}\}}} + n_{D,t}^{(b)}}$in which h_(S) _(i) _(,D,t)ϵC represents the channel gain between thesource S_(i) and the destination D, h_(R,D,t)ϵC represents the channelgain between the relay R and the destination D, n_(D,t) ^((b))ϵC^(N)^(b) is a noise vector, and l_({J) _(R,t) _(b) _(=ϕ}) is an indicator toindicate whether or not the relay is silent:

$1_{\{{J_{R,t}^{b} \neq \phi}\}} = \left\{ \begin{matrix}1 & {{{if}\mspace{14mu} J_{R,t}^{b}} \neq \phi} \\0 & {else}\end{matrix} \right.$where ϕ represents the empty set.

The destination may use “sliding window” decoding, or it may use“backward” decoding.

When using a sliding window of length Q, Q ϵ{1, . . . , T}, thedestination begins at the end of each time slot t ϵ{Q, . . . , T} todecode messages from the sources in order to extract the QK informationbits from each source that are transmitted during the time slots t, t−1,. . . , t−(Q−1) with help from the message from the relay as transmittedduring the time slots t, t−1, . . . , t−(Q−1).

The size of the sliding window may be set at a maximum value, and thedestination can decide on the optimum value on the basis of thesignalling coming from the relay. For example, if the maximum size isthree and if the destination knows that the relay does not give help toany source during the first time slot in the absence of messages thathave been decoded without error, then the destination can begin todecode the first time slot immediately without waiting for three timeslots.

The size of the sliding window at the destination can have an impact onthe selection scheme of the relay. During a given time slot t, theselection scheme may be such that the relay does not give help to themessages that have been decoded without error in the time slotspreceding the time slot t−(Q−1), i.e. the messages decoded during thetime slots ϵ{1, . . . , t−Q}.

FIG. 13 shows the structure of the decoder at the destination in theform of a factor graph during the sliding window Q that covers all ofthe time slots from t−(Q−1) to t. Thus, for each time slot covered bythe window, the detection and decoding systems of the sub blocks b, b=1,. . . B, received from the sources are activated in succession, e.g. byoperating switches to make the systems active. And, for each time slotcovered by the window, the systems for decoding sequences received fromthe relay are activated, e.g. by operating switches in order to make thesystems active as a function of signalling information giving each subslot, i.e. each block, an indication as to whether or not it has beentransmitted by the relay. All of the blocks and messages received duringthe window Q are decoded jointly. The controlled connection matrix andnetwork interleavers sub unit makes connections between the variablenodes that represent the messages from the sources and the variablenodes that represent the sequences generated by the relay at each subslot of each time slot. As input, this sub unit receives signallinginformation coming from the relay indicating for each sub slot theselection scheme used by the relay. The beliefs in the form of LLRsabout the various variable nodes are exchanged within the factor graphuntil achieving convergence.

When implementing so-called “backward” decoding, the destinationattempts to decode the messages from the sources at the end of time slott=T, in order to extract the TK information bits from each source. Thisdecoding scenario is appropriate when the T messages from a source forma frame (or super message), which means that there is no point indecoding one message before decoding the other messages. In contrast,when the messages from a source are independent, this scenario givesrise to pointless latency by necessarily waiting for the end of the timeslot t=T. An example of the structure of the decoder is shown in FIG.13. It is easy to obtain the “backward” decoding situation by settingt=T and Q=T. The constraint node MUD_(t) ^((b)) is linked to thevariable nodes b_(S) ₁ _(,t) ^((b)), . . . , b_(S) _(M) _(,t) ^((b)),b_(R,t) ^((b)) and to the observation y_(D,t) ^((b)). For reasons ofclarity, we have duplicated the constraint node MUD_(t) ^((b)) for eachvariable node. Nevertheless, a single activation of detecting multipleusers associated with the constraint node MUD_(t) ^(b) makes it possibleto generate the beliefs (or extrinsic LLRs) to all of the associatedvariable nodes by taking account of the observation y_(D,t) ^((b)) andthe incident beliefs.

The decoder of FIG. 13 may be run as follows.

Step 1, all of the multiuser detectors MUD are activated as a functionof the received signal:y _(D,t−(Q−1)) ⁽¹⁾ , . . . ,y _(D,t−(Q−1)) ^((B)) , . . . ,y _(D,t) ⁽¹⁾, . . . ,y _(D,t) ^((B)).

Step 2, the variable nodes representing the network coded messagescoming from the relay receive the beliefs about their values byactivating their interleavers and corresponding SISO modules.

Step 3, the variable nodes representing the messages from the sourcesreceive the beliefs about their values by activating firstly theirinterleavers and corresponding SISOs, and secondly the controlledconnection matrix and network interleavers sub unit.

Step 4, on the basis of the beliefs received from the variable nodes ofthe messages from the sources, the destination verifies whether themessages have or have not been correctly decoded. If they have u_(S) ₁_(t−(Q−1)), u_(S) ₂ _(,t−(Q−1)) . . . , u_(S) _(M) _(,t−(Q−1)) beencorrectly decoded, the destination stops decoding iterations on thissliding window and slides the window to the following window (fromt−(Q−1)+1 to t+1). Otherwise, the method moves on to step 5.

Step 5, the variable nodes representing the network coded messages fromthe relay receive the beliefs about their values from the controlledconnection matrix and network interleavers sub unit.

Step 6, the multiuser detectors MUD receive the beliefs about the bitsforming the modulated signals y_(D,t−(Q−1)) ⁽¹⁾, . . . , y_(D,t−(Q−1))^((B)), . . . , y_(D,t) ⁽¹⁾, . . . , y_(D,t) ^((B)); these beingobtained as from activation of the interleavers and SISO unitsassociated with the network coded messages and with the messages fromthe sources.

Steps 1 to 6 are repeated until reaching a maximum number of iterationsor until leaving the loop during step 4.

It should be observed that a plurality of messages at time slots otherthan t−(Q−1) can be decoded without error. These are used forsimplifying decoding for the next decoding windows.

The number of decoding windows used governs the complexity of thereceiver, so in certain applications it is possible for priority to begiven to only a few decoding windows.

The destination may form part of a plurality of MARC systems that sharethe same sources and the same destination, but that have differentrelays, these relays using mutually different radio resources.

In the presence of second relays R′, the constraint node MUD_(t) ^((b))is linked to the variable nodes b_(S) ₁ _(,t) ^((b)), . . . , b_(S) _(M)_(,t) ^((b)), b_(R,t) ^((b)), b_(R′,t) ^((b)) and to the observationy_(D,t) ^((b)). As a result, the left-hand portion of FIG. 13 isduplicated, replacing R with R′. Activating the detection of multipleusers associated with the constraint node MUD_(t) ^((b)) makes itpossible to generate the beliefs (or extrinsic LLRs) to all of theassociated variable nodes b_(S) ₁ _(,t) ^((b)), . . . , b_(S) _(M) _(,t)^((b)), b_(R,t) ^((b)), b_(R′,t) ^((b)) by taking account of theobservation y_(D,t) ^((b)) and the incident beliefs.

By way of example, the sources are users who seek to transmit messagesto a common destination, e.g. a base station of a mobile access network.The sources are helped by a relay, which may be a cut-down base station,or for example one of the sources, when that source lies on the pathbetween the sources and the base station. The sources may equally wellbe base stations addressing the same destination.

In a particular use, the two sources correspond to two mobile terminals.In another use, the two sources may correspond to two different servicesthat are accessible from a single terminal, but under such circumstancesthe terminal is provided with at least two antennas that determine twodifferent propagation channels between the terminal and the relay, andbetween the terminal and the destination.

APPENDIX

-   LLR: “Log Likelihood Ratio”-   for U being a random binary variable, its log likelihood ratio (LLR)    is defined by the following relationship:

${LLR}_{u} = {\log\left( \frac{P_{U}\left( {u = 1} \right)}{P_{U}\left( {u = 0} \right)} \right)}$in which relationship P_(U) (u) denotes the probability that the randomvariable U takes the value u.

-   LAPPR: “Log A Posteriori Probability Ratio”

Represents to the LLR conditional on an observation that generallycorresponds to a received signal.

${LAPPR}_{u} = {\log\left( \frac{P\left( {u = {1❘y}} \right)}{P\left( {u = {0❘y}} \right)} \right)}$

-   [1] F. R. Kschischang, B. J. Frey, and H.-A. Loeliger, “Factor    graphs and the sum-product algorithm,” IEEE Trans. Inform. Theory,    vol. IT-47, no. 2, pp. 498-519, February 2001.

The invention claimed is:
 1. A relaying method performed by afull-duplex relay for a telecommunications system comprising a pluralityof sources, the full-duplex relay, and a destination, the methodcomprising: a decoding process comprising estimating for each source acode word x_(S,t) from received blocks corresponding to successive codewords x_(S,t) transmitted simultaneously by the plurality of sourcesduring T time slots, such that, at each time slot t, t=1, . . . , T, thecode word x_(S,t) comprises B blocks x_(S,t) ⁽¹⁾, x_(S,t) ⁽²⁾, . . . ,x_(S,t) ^((B)), a first block of the B blocks to be decodedindependently of the other blocks, a code word x_(S,t) encoding a K-bitmessage u_(S,t), where K, B, and T are natural integers, B>2, T>1;detecting, by the full-duplex relay, messages that have been decodedwithout error and taking a decision; and encoding a signal andtransmitting it to the destination, wherein the signal is representativeonly of messages that have been decoded without error; wherein, aftereach received block, the encoding and transmission are performed underthe control of the process of detecting messages that have been decodedwithout error and of taking a decision in compliance with a selectionscheme, and wherein the selection scheme is such that a representativesignal relating to a message that has been decoded without error duringa time slot is transmitted by the full-duplex relay during the same timeslot.
 2. A method according to claim 1, further comprising storingmessages that have been decoded without error.
 3. A method according toclaim 1, wherein in the absence of a message that has been decodedwithout error during a current time slot t, the error detection anddecision taking process allows a message that has been decoded withouterror during a preceding time slot to be encoded and transmitted.
 4. Amethod according to claim 1, wherein the decision allows a message to beencoded and transmitted as soon as it has been decoded without error. 5.A method according to claim 1, wherein after each received block,detection and decoding are under the control of the process of detectingmessages that have been decoded without error and of taking a decisionin compliance with a selection scheme.
 6. A method according to theclaim 5, wherein if all the messages are decoded without error during acurrent time slot t, then the process of detecting messages that havebeen decoded without error and of taking decisions stops detection anddecoding until the end of the current time slot and allows encoding andtransmission.
 7. A method according to claim 1, wherein the selectionscheme is such that, at each current time slot t, the transmitted signalis representative of the messages decoded without error up to thecurrent time slot t.
 8. A method according to claim 1, wherein theselection scheme is such that, during each current time slot t, thetransmitted signal is representative of the messages that have beendecoded without error solely during the current time slot t.
 9. A methodaccording to claim 1, wherein the encoding and transmission processcomprises interleaving for each source for which a message has beendecoded without error prior to network coding.
 10. A method according toclaim 1, wherein the encoding and transmission process comprises networkcoding followed by first interleaving, by channel coding, and by secondinterleaving distinct from the first.
 11. A full-duplex relay for atelecommunications system comprising a plurality of sources, thefull-duplex relay, and a destination, the full-duplex relay comprising:a decoder configured to estimate for each source a code word x_(S,t)from received blocks corresponding to successive code words x_(S,t)transmitted simultaneously by the plurality of sources during T timeslots, such that, at each time slot t, t=1, . . . , T, the code wordx_(S,t) comprises B blocks x_(S,t) ⁽¹⁾, x_(S,t) ⁽²⁾, . . . , x_(S,t)^((B)) , a first block of the B blocks to be decoded independently ofthe other blocks, a code word x_(S,t) encoding a K-bit message u_(S,t),where K, B, and T are natural integers, B>2, B>2; a decision componentconfigured to enable the full-duplex relay to detect messages that havebeen decoded without error and to take a decision; and an encoder andtransmitter configured to encode a signal and transmit it to thedestination, wherein the signal is representative only of messages thathave been decoded without error; wherein the full-duplex relay isconfigured such that, after each received block, the encoder andtransmitter is under the control of the decision component which detectsmessages that have been decoded without error and which takes a decisionin compliance with a selection scheme, and wherein the selection schemeis such that a representative signal relating to a message that has beendecoded without error during a time slot is transmitted by thefull-duplex relay during the same time slot.
 12. A reception method fora receiver of at least one MARC system for performing a relaying methodaccording to claim 1, wherein the method comprises: jointly detectingand decoding blocks coming from the sources and of messages coming fromthe full-duplex relay, the detection and decoding being performediteratively at the end of each time slot t such that t ϵ{Q, . . . , T}over the duration of a sliding window of length Q, Q ϵ{1, . . . , T},with propagation of probabilities between the iterations in order toestimate Q messages for each source; wherein the decoding of themessages from the full-duplex relay is configured in each sub slot onthe basis of signalling information coming from the full-duplex relayindicating whether the full-duplex relay is cooperating on this block b,b=1, . . . B; and wherein the propagation of probabilities is configuredfor each block b, b=1, . . . B, on the basis of signalling informationcoming from the full-duplex relay indicating the selection scheme of thefull-duplex relay, with B and T being natural integers, B>2, T>1.
 13. Areceiver for at least one multiple-access relay channel (MARC) system,said receiver comprising: a detector and decoder configured to jointlydetect and decode blocks coming from a plurality of sources and messagescoming from a relay, the detection and decoding being performediteratively at the end of each time slot t such that t ϵ{Q, . . . , T}over the duration of a sliding window of length Q, Q ϵ{1, . . . , T},with propagation of probabilities between the iterations in order toestimate Q messages for each source of the plurality of sources; whereinthe receiver is configured such that the decoding of the messages fromthe relay is configured in each sub slot on the basis of signallinginformation coming from the relay indicating whether the relay iscooperating on this block b, b=1, . . . B; and such that the propagationof probabilities is configured for each block b, b=1, . . . B, on thebasis of signaling information coming from the relay indicating theselection scheme of the relay, with B and T being natural integers, B>2,T>1.
 14. A method of transmitting a digital signal for atelecommunications system comprising a plurality of sources, a relay,and a destination implementing a spatially distributed network code, themethod comprising for each source: encoding messages u_(S,t) includingrespective CRCs into code words c_(s,t); and transmitting the code wordsc_(s,t) during T time slots to the relay and to the destination; whereinthe encoding is of finite incremental redundancy type and, at each subslot b=1, 2, . . . , B of a time slot, it delivers a block c_(S,t)^((b)) such that the B successive blocks {c_(S,t) ^((b)): 1≤b≤B} form acode word c_(s,t) of the code words, a first block of the B blocks to bedecoded independently of the other blocks, such that the followingblocks are parity bits that add redundancy to the first block, with Band T natural integers, B>2, T>1.
 15. A transmitter of a digital signalfor a telecommunications system having a plurality of sources, a relay,and a destination implementing a spatially distributed network code,comprising: an encoder which encodes messages u_(S,t) having respectiveCRCs into code words c_(s,t); and a transmitter which transmits the codewords c_(s,t) during T time slots to the relay and to the destination;wherein the encoder is of finite incremental redundancy type and, ateach sub slot b=1, 2, . . . B of a time slot, it delivers a blockc_(S,t) ^((b)) such that the B successive blocks {c_(S,t) ^((b)): 1≤b≤B}form a code word c_(s,t)of the code words, a first block of the B blocksto be decoded independently of the other blocks, such that the followingblocks are parity bits that add redundancy to the first block, with Band T natural integers, B>2, T>1.